Sorry I am not sure what a cube map is, but here is an image of what I am referring to. The left image has the overlay on the background image but the right one does not, which makes the background image look a lot more vibrant.
[media]I have noticed this issue as well. The game will not hit 60 fps unless deep underground. On land it averages at 40 with frequent drops to the 30s. Playing on Xbox One.
This report is valid. Sand had darker shading in 1.21.50.24 Preview, but no longer does in 1.21.50.25 Preview. That makes it valid to report. You claim darker sand shading is unintentional, but that is an assumption.
I believe it is more likely that Mojang intentionally changed the sand shading to match Java Edition, as they did this with other blocks as well, but since the ambient_occlusion_exponent was not functional, they changed the hardcoded shading values instead. The problem is that some blocks had the ambient_occlusion_exponent field applied to them, so when they made the field functional in 1.21.50.25 Preview, it unintentionally reverted their changes for some blocks.
Ultimately, Mojang should be the one to decide whether this is intentional or not.